The present invention relates generally to the field of electronic circuits and, in particular, to a phase detector.
Phase detectors detect or measure the relative phase of two signals with respect to each other, and are used in a wide variety of electronic systems. For example, a phase detector is a fundamental building block for a phase lock loop (PLL) found in many electronic systems. A PLL is particularly useful in demodulating radio frequency (RF) signals in, for example, an FM radio receiver.
A PLL is a circuit that causes a particular system to track with another system. More particularly, a PLL is a circuit that synchronizes an output signal (generated by an oscillator) with a reference or input signal in frequency as well as in phase. A typical PLL includes three main building blocks: a phase detector, a loop filter and a voltage (or current) controlled oscillator. The phase detector receives the reference or input signal as well as the output of the voltage controlled oscillator. The phase detector measures the phase difference between the input signal and the output signal of the voltage controlled oscillator. The phase difference acts as an error signal that is fed to the voltage controlled oscillator via the loop filter. When, the phase detector detects zero, or very small, phase error between the input or reference signal and the output of the oscillator, the PLL is said to be locked.
Common types of phase detectors include analog multiplier circuits such as the Gilbert cell and ring diode mixer topologies. These phase detectors typically accept sinusoidal input signals. Other phase detectors accept digital input signals. For example, exclusive OR gate and RS Flip-Flop phase detectors fall into this category. The detectors produce a duty cycle modulated output whose average value is proportional to the phase difference. A last type of detector is the Sequential Phase/Frequency Detector. This type of detector produces two outputs, the first (second) labeled as up (down). These two outputs are individually duty cycle modulated depending on which input is leading and the magnitude of the phase difference.
Phase detectors are used in a number of conventional applications requiring continuous measurement of phase error control, e.g., a Voltage Controlled Oscillator (VCO). Other applications include using a phase detector to measure the change in phase in a Phase Shifted Keying (PSK) communications system where the digital data is encoded in the phase of the transmitted signal. These examples show applications where the measurement of phase is important, but not necessarily the precise measurement of phase. An application, which requires a precise measurement of phase, is a Transit Time flow meter.
A Transit Time flow meter estimates volumetric flow by measuring the phase difference between bursts of ultrasound traveling upstream, and downstream paths across a tube with moving fluid. The phase difference is dependent on the volumetric flow when the entire tube or vessel is illuminated with the sound waves. Papers published by Craig Hartley, Ph.D., or Cor Drost, Ph.D., explain that the moving fluid causes the time required by the sound waves to travel across the vessel to be different for an upstream and downstream path when the fluid is moving. In other words, when the same signal is transmitted on the upstream and downstream paths, a phase difference is introduced between the two received signals by the motion of the fluid.
Transonics Systems Inc., a commercial supplier of Transit-Time flow measurement equipment, measures the phase shift with an analog multiplier. This multiplies the received ultrasound signal with the signal from a master oscillator and measures the phase difference between the two input signals. The measurement cycle is repeated on the opposite direction and the phase measurements are subtracted to produce the phase shift between the upstream and downstream paths. The phase difference measured is then proportional to the volumetric flow at that point in time. A limitation of this phase detection method requires a long burst of ultrasound be transmitted from one transducer to the other, along the upstream or downstream path, with a duration long enough to allow the analog multiplier and the low pass filter time to settle on the phase value.
Crystal Biotech, Inc. (CBI) uses another method to measure phase shifts in a Transit Time flow meter created by Ronald Patton. CBI simultaneously transmits a short burst of ultrasound from two transducers in a probe and compares the phase shift of the received ultrasound bursts from the upstream and downstream paths directly to each other. The CBI Transmit Time flow meter includes a digital circuit with a single output. This output signal has its duty cycle modulated by the phase difference. This single modulated output switches on and off a current source with a capacitor as its load with a selectable number of pulses. The current source is switched on and off and the capacitor is used to store the charge, which is proportional to the time the current source is on. The charge on the capacitor generates a voltage, which is proportional to the phase shift between the two input signals. One shortcoming of the CBI device is that the portion of the signal representing the change in phase is a small percentage of the total charge on the capacitor. Therefore, it is difficult to reliably measure the small phase changes generated by the CBI Transit Time flow meter.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a phase detector that provides a more rapid and accurate measure of the phase difference between two signals.
The above mentioned problems with phase detectors and other problems are addressed by the present invention and will be understood by reading and studying the following specification. A phase detector is described which modulates the duty cycle of first and second output signals with a phase difference between the first and second input signals and uses the duty cycle modulated signals to create a differential signal that is proportional to the phase difference between the first and second input signals. For example, in one embodiment, the phase detector creates the differential voltage by charging first and second capacitors through switches controlled by the duty cycle modulated first and second output signals.